Secondary ion mass spectrometry (SIMS) has become a purity analysis method of choice for surface and near surface investigation of solid samples because of its ability to provide parts-per-million to parts-per-billion sensitivity and excellent depth resolution. SIMS relates to a technique for surface and near surface analysis which involves ion bombardment of the sample surface for depth profiling. A good tutorial relating to the SIMS technique can be found on the Internet at http://www.cea.com/tutorial.htm, which is incorporated by reference in its entirety.
As discussed therein, SIMS involves the use of a bombarding primary ion beam onto a sample. Primary ions are implanted and mix with sample atoms to depths of 1 to 10 nm depending on the bombarding energy. The bombarding ion produces monatomic and polyatomic neutrals and ions of sample material, resputtered primary ions, as well as electrons and photons. Sputter rates depend on primary beam intensity, sample material, and crystal orientation. Sputter rates in typical SIMS analyses vary between 0.5 and 5 nm/s.
The primary ion beam species that are typically useful in SIMS analysis include Cs.sup.+, O.sup.2+, O.sup.-, Ar.sup.+, and Ga.sup.+ at energies between 1 and 30 keV.
SIMS instruments include both static SIMS instruments, e.g., time-of-flight instruments, and dynamic SIMS instruments. Dynamic SIMS instruments in turn primarily employ use two kinds of mass analyzers, magnetic sector and quadrupole analyzers.
Magnetic sector instruments are common in the field. In these instruments, the ion beam passes through the magnetic field where the particles are acted on by a force at right angles, both to the direction of motion and to the direction of the magnetic field. Modern mass spectrometers use non-normal pole faces for entrance and exit of the ion beam to the magnetic sector. The fringings fields in this configuration compress the ion beam in the vertical direction (in and out of the screen) as it passes through the sector. Fewer ions strike metal surfaces and the ion beam focuses better at the exit slit with non-normal pole faces. The entrance and exit slits can be arranged at ion beam crossovers for the cleanest separation (highest mass resolution) between ions with similar m/z values.
Quadrupole mass analyzers, on the other hand, have been employed in many kinds of analysis since their invention in 1953. Quadrupole analysis employ rods that ideally have hyperbolic shapes, but this geometry can be approximated with closely spaced circular rods. In a typical quadrupole spectrometer, the rods are 1 cm in diameter and 20 cm long. Ions enter at a relatively low energy (.about.25 eV). Alternating and direct voltages on the rods cause the ions to oscillate after entering the quadrupole. For a given set of voltages, ions with a single mass-to-charge ratio undergo stable oscillation and traverse through the rods. All other ions have unstable oscillations and strike the rods. The alternating frequency and the ratio between the alternating and direct voltages remain constant.
The widespread use of SIMS technology can be seen, for example, from the article "Secondary Ion Mass Spectrometry--First Microelectronics, Now the Rest of the World", by F. A. Stevie, Surface and Interface Analysis, Vol. 18, 81-86 (1992), which is incorporated by reference in its entirety.
The use of SIMS in connection with insulating materials is also known. However, it has also suffered from a number of problems. As shown in FIG. 1, ion bombardment 1, of an insulating surface 2, induces a positive charge 3, onto the oxide surface. This problem has been addressed by a variety of approaches, the most common of which involves the use of an electron beam 4, that seeks to neutralize the positive ion buildup. Electron beam neutralization has been used in connection with both magnetic sector and quadrupole based SIMS instruments.
Despite the effectiveness of electron beam neutralization in connection with quadrupole and magnetic sector instruments, there are nonetheless significant limitations on the use of SIMS in certain environments. One specific area in which SIMS analysis, and in particular magnetic sector based SIMS analysis, has not been effective involves the determination of alkali elements in insulators.
This problem is not unknown in the art and as was discussed, for example, in "Depth Profiling of Sodium in SiO.sub.2 Films by Secondary Ion Spectrometry", C. W. Magee et al., Applied Physics Letters, 33, 193 (1978).
Traditional SIMS analysis of alkali metals in insulators has been deficient because the alkali elements will, in essence, "move" when an electric field is applied during SIMS testing. Such movement provides a false negative, i.e., will appear to indicate that the alkali metals are not present when in fact they are present in the insulator or, alternatively, will not show the proper location of the alkali elements.
The ability to accurately determine the presence of alkali elements as well as their quantity and exact location in insulators is particularly critical in the manufacture of semiconductor materials. The movement of alkali elements in the insulator under application of an electric field results in an insulator that acts as a conductor. Thus, insulators including alkali elements are largely useless in the field of electronic devices.
The primary technique that the art has employed in order to minimize this problem has involved the introduction of a conductive film coating onto the surface of the insulator. However, such a coating can introduce significant contamination problems during fabrication. Thus, it is an unacceptable solution to the problem of alkali metal movement in the field of semiconductor manufacture.
Accordingly, the need still exists for a method for accurately analyzing alkali elements in insulators.